Ceramic arc tube assembly and method of making a ceramic arc tube

ABSTRACT

A ceramic arc tube assembly and a method for making a ceramic arc tube are described which simplify the manufacture of ceramic arc tubes by reducing the number of handling and heat treatments required to assemble arc tubes prior to the final sintering operation. In particular, the invention uses transient assembly buttons during intermediate assembly steps which are removed prior to the final sintering operation.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/271,153, filed Feb. 23, 2001.

TECHNICAL FIELD

[0002] This invention relates to ceramic arc tubes and more particularlyto ceramic arc tube assemblies and methods of making such assemblies.

BACKGROUND OF THE INVENTION

[0003] Over the years ceramic arc tubes composed of materials such aspolycrystalline alumina have been used to contain the discharges ofhigh-pressure sodium lamps. This has resulted in the development of anumber of arc tube configurations for use in sodium lamps. For example,U.S. Pat. No. 4,766,347 describes a three-piece arc tube configurationwherein the arc tube comprises a ceramic body with tubular closuremembers. The closure members which receive the electrodes are sealeddirectly in the ends of the ceramic body. In another example, U.S. Pat.No. 5,426,343 describes a three-piece arc tube configuration whereinend-sealing buttons are used which have an extending electrode-receivingmember integral therewith.

[0004] More recently, ceramic arc tubes have been employed in metalhalide lamps. For example, U.S. Pat. No. 5,424,609 describes afive-piece ceramic arc tube configuration for a metal halide lamp. Thefive-piece arc tube includes a cylindrical body, a pair of end-sealingbuttons, and a pair of capillary tubes sealed to the buttons. Themanufacture of these ceramic arc tubes requires extrusion or pressing ofthe individual components, as well as multiple assembly and heattreatment steps. These multiple steps result in increased handling whichincreases the manufacturing cost.

SUMMARY OF THE INVENTION

[0005] It is an object of the invention to obviate the disadvantages ofthe prior art.

[0006] It is another object of the invention to provide a ceramic arctube assembly which facilitates and simplifies the manufacturing ofceramic arc tubes.

[0007] It is another object of the invention to provide a method formaking ceramic arc tubes which reduces the number of handling and firingsteps in the manufacturing process.

[0008] In accordance with one object the invention, there is provided aceramic arc tube assembly comprising a hollow body having at least oneopen end containing an end cap, the end cap having a capillary tube anda transient assembly button, the capillary tube extending outwardly fromthe hollow body and having a length inserted into the open end andforming a seal with the hollow body, the transient assembly button beingfixed around the capillary tube and contacting an edge of the open end,the interaction between transient assembly button and the edge of theopen end determining the length of the capillary tube inserted into theopen end, and the transient assembly button being capable of removalfrom the capillary tube without causing damage to the arc tube assembly.

[0009] In accordance with another object of the invention, there isprovided a method of making a ceramic arc tube comprising the steps of:

[0010] (a) fixing a transient assembly button around a capillary tube toform an end cap;

[0011] (b) inserting the end cap into an open end of a hollow arc tubebody until the transient assembly button contacts an edge of the openend;

[0012] (c) heating the assembly to form a mechanical seal between thecapillary tube and the open end of the hollow body;

[0013] (d) removing the transient assembly button without damaging theassembly; and

[0014] (e) sintering the assembly to form the ceramic arc tube.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a cross-sectional view of an end cap for a three-piececeramic arc tube assembly.

[0016]FIG. 2 is a cross-sectional view of a three-piece ceramic arc tubeassembly.

[0017]FIG. 3 is a cross-sectional view of a completed ceramicthree-piece arc tube.

[0018]FIG. 4 is a cross-sectional view of an end cap for a five-piececeramic arc tube assembly.

[0019]FIG. 5 is a cross-sectional view of a five-piece ceramic arc tubeassembly.

[0020]FIG. 6 is a cross-sectional view of a completed ceramic five-piecearc tube.

[0021]FIG. 7 is a cross-sectional view of another embodiment of afive-piece ceramic arc tube assembly.

[0022]FIG. 8 is an illustration of a grooved transient assembly button.

[0023]FIG. 9 is an illustration of a notched transient assembly button.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0024] For a better understanding of the present invention, togetherwith other and further objects, advantages and capabilities thereof,reference is made to the following disclosure and appended claims takenin conjunction with the above-described drawings.

[0025] The ceramic arc tube assembly and method of this inventionsimplify the manufacture of ceramic arc tubes by reducing the number ofhandling and heat treatments required to assemble the arc tube prior tothe final sintering operation. In one embodiment, the ceramic arc tubeassembly comprises a hollow arc tube body having at least one open endcontaining an end cap. The end cap comprises a transient assembly buttonand a capillary tube for receiving an electrode. The transient assemblybutton is fixed around the capillary tube by an initial heat treatmentwhich causes the button to shrink and compress against the capillarytube. When the components are being assembled, the transient assemblybutton interacts with the edge of the open end to limit the penetrationof the end cap into the arc tube body. This interaction fixes the lengthof the capillary tube inserted into the arc tube body. After thecomponents are assembled, the end cap is then joined to the arc tubebody in a second heat treatment step which mechanically seals the endcap to the arc tube body. The completed arc tube is then made byremoving the transient assembly button and firing the assembled arc tubein a final sintering operation.

[0026] In another embodiment, the capillary tube has aradially-extending sealing member which is inserted into the open end ofthe arc tube body and sealed therein. The sealing member may comprise aseparate part which is joined to the capillary tube along with thetransient assembly button in the initial heat treatment or it may beformed as an integral part thereof. Preferably, the transient assemblybutton has the form of an annular disc whereby the capillary tube isinserted into the center aperture. However, other shapes may also beequally effective provided that the transient assembly button can beremoved prior to the final sintering operation without damaging the arctube assembly.

[0027] When the both ends of the arc tube require end caps to be sealedtherein, the use of the transient assembly buttons is particularlyadvantageous as it allows both ends of the arc tube assembly to besealed simultaneously in a vertical orientation. This simplifies theassembly process by eliminating the need to seal each end of the bodyportion to an end cap in two sequential heat treatment operations at twodifferent temperatures.

[0028] The ceramic arc tube assembly of the present invention ispreferably formed from polycrystalline alumina containing minor amountsof magnesia and, in some instances, yttria and zirconia. Such a materialis shown in U.S. Pat. No. 5,682,082. Other minor constituents may alsobe included as is known in the art. In a preferred embodiment, the endcaps of the arc tube assemblies (capillaries, transient assemblybuttons, sealing members) are made of Baikowski grade CR-6 aluminapowder containing 0.05 weight percent magnesia and the body portion ofeither Baikowski grade CR-6 or Baikowski grade CR-30 powder containing0.05 weight percent magnesia. The components of the ceramic arc tubeassembly may be formed by a number of conventional methods, e.g.,extrusion, pressing, or injection molding.

[0029] The relative positions of the components in the arc tube assemblyare fixed in a heat treatment step at or below about 1350° C. The heattreatment causes the components to densify and shrink resulting incompressive forces which fix the positions of the components. Because ofthe relatively low temperature of the heat treatment step, there is nocross diffusion or grain growth between the components to bond themtogether chemically. In addition, at temperatures below 1350° C., thetransient assembly buttons retain considerable porosity which limittheir strength. This allows the transient assembly buttons to be easilysnapped off of the capillary tubes without damaging the arc tubeassembly.

[0030] FIGS. 1-3 illustrate the various stages of manufacture for athree-piece ceramic arc tube using the invention described herein. InFIG. 1, an end cap 3 comprised of capillary tube 2 and transientassembly button 11 is formed by placing capillary tube 2 through a holein perforated ceramic plate 5 (shown in dotted line) of controlledthickness L. The perforated ceramic plate 5 rests on solid ceramic plate10 (also shown in dotted line) and prevents the capillary tube frompassing completely through the perforated plate. Transient assemblybutton 11 is placed on the capillary tube 2 and is supported by theperforated ceramic plate 5. The plates and the supported end capcomponents are then placed in a furnace and heated in air at from about1200° C. to about 1250° C. This initial heat treatment causes transientassembly button 11 to shrink and become fixed around the capillary tube.The thickness L of the perforated ceramic plate corresponds to thelength of capillary tube 2 which will be inserted into the arc tubebody.

[0031] In FIG. 2, an arc tube assembly is made by placing the capillary2 a of an end cap 3 a through a hole in perforated ceramic plate 5(shown in dotted line) with the protruding end of the capillary 2 aextending below the plate and transient assembly button 11 a supportedon the plate. A hollow elliptical body portion 4 having open ends 7 a, 7b is placed over the capillary portion of the end cap 3 a. A second endcap 3 b is inserted into the opposite end 7 b of the hollow body. Theperiphery 17 a, 17 b of the transient assembly buttons 11 a, 11 b ofeach end cap 3 a, 3 b must be greater than the inside diameter of theopen ends 7 a, 7 b just prior to assembly. The length of capillary tubes2 a, 2 b inserted into the open ends 7 a, 7 b of the hollow body isdetermined by the interaction between the edges 13 a, 13 b of the openends and the transient assembly buttons 11 a, 11 b. It is preferred thatafter assembly the periphery 17 a, 17 b of the transient assemblybuttons 11 a, 11 b be greater than the periphery 19 a, 19 b of the openends 7 a, 7 b to facilitate removal of the transient assembly buttons.Preferably, the transient assembly buttons extend about 1 mm beyond theperiphery of the open ends. The assembled components are then heattreated in air at from about 1250° C. to about 1350° C. during which thebody portion densities sufficiently to shrink onto the capillary tubesforming mechanical seals 9 a, 9 b and a completed arc tube assembly.

[0032] The transient assembly buttons are then removed from thecapillary tubes without damaging the arc tube assembly. The assembly canbe fired either horizontally, or, in the preferred method, vertically ina final sintering operation at a temperature above about 1800° C. in ahydrogen-containing atmosphere. Preferably, the final sintering isperformed at about 1880° C. for about 180 minutes in 100% hydrogen. Thefinished arc tube is shown in FIG. 3. The final sintering operationpromotes grain growth and interdiffusion at the existing mechanicalseals in the arc tube assembly in combination with further shrinkage.These actions cause the formation of hermetic seals 9 a′, 9 b′ in thesintered assembly which are both vacuum and gas tight.

[0033] FIGS. 4-6 illustrate the various stages of manufacture for afive-piece ceramic arc tube using the invention described herein. InFIG. 4, the end cap 23 is shown comprised of a capillary tube 2, sealingmember 25 and transient assembly button 11. The periphery 17 oftransient assembly button 11 extends beyond the periphery 21 of sealingbutton 25. The end cap components are assembled on a ceramic plate (notshown) and heat treated in air at from about 1200° C. to about 1250° C.During this treatment, sealing member 25 and transient assembly button11 densify sufficiently to shrink onto capillary 2 applying acompressive force which fixes the relative positions of the components.

[0034] The arc tube assembly shown in FIG. 5 is made by inserting thecapillary 2 a of end cap 23 a through a hole in perforated ceramic plate5 (shown in dotted line) with the protruding end of the capillary 2a-extending below the plate and transient assembly button 11 a supportedon the plate. A cylindrical hollow arc tube body 24 having opposed openends 27 a, 27 b is placed over the sealing member 25 a of end cap 23 a.

[0035] A second end cap 23 b is inserted into the opposite open end 27 bof the cylindrical body portion. The periphery 17 a, 17 b of transientassembly button 11 a, 11 b must be greater than the inside diameter ofthe open ends 27 a, 27 b of the cylindrical body just before assembly.The interaction between the transient assembly buttons 11 a, 11 b andthe edges 33 a, 33 b of open ends 27 a, 27 b determines the insertionlength of the capillary tubes 2 a, 2 b. It is preferred that afterassembly, the periphery 17 a, 17 b of the transient assembly buttons 11a, 11 b extend beyond the outside diameter of the open ends 27 a, 27 bto facilitate removal of the transient assembly buttons.

[0036] The arc tube assembly is then heat treated in air at from about1250° C. to about 1350° C. to cause the cylindrical body to shrink ontothe sealing members 25 a, 25 b to form mechanical seals 29 a, 29 b. Thetransient assembly buttons are then removed from the capillary tubes andthe assembly is subjected to the final sintering to form a finished arctube as shown in FIG. 6.

[0037] Another embodiment of a five-piece arc tube assembly of thisinvention is shown in FIG. 7, this five-piece arc tube assembly issimilar to the one shown in FIG. 5 except that the bottom end cap 23 ais not made with a transient sealing button. The end cap 23 b whichutilizes transient assembly button 11 is inserted only into the upperopen end 27 b of the hollow body 24. The insertion length of capillary 2b is determined by the interaction between the transient assembly button11 and edge 33 b whereas the insertion length of capillary 2 a isdetermined by the interaction between the surface of perforated plate 5(shown in dotted line) and edge 33 a. After a heat treatment tomechanically seal sealing members 25 a, 25 b in open ends 27 a, 27 b,the single transient assembly button is snapped off the upper capillarytube 2 b to form a completed assembly.

[0038] In another embodiment, capillary tubes 2 may be subjected to athermal pre-treatment prior to being joined with transient assemblybuttons 11 or sealing members 25. The thermal pretreatment densities thecapillary tubes causing them to shrink so that they may fit throughopenings in the transient assembly buttons or sealing members. Thepretreatment may be performed in air at a temperature from about 1250°C. to about 1350° C.

[0039] To better facilitate removal of the transient assembly buttons,it is often desirable to incorporate a groove or notch in the buttons toreduce their strength further. The groove in the transient assemblybutton is typically made on one side to a depth of from about 50 percentto about 75 percent of the button thickness. The groove can be producedby a feature in the button die pressing tooling or cut by an abrasivesaw. A transient assembly button comprising a grooved annular disc isshown in FIG. 8. The groove 58 is cut along a diameter of the button 51.When using grooved transient assembly buttons, it is preferred thatduring the end cap assembly the groove be oriented so that it will facethe open end of the arc tube body when the end cap is inserted. Thisorientation allows the groove to be under maximum tensile stress duringremoval.

[0040] A second method to reduce the strength of the transient assemblybutton is to notch the button. In a preferred embodiment shown in FIG.9, the notch 68 extends half-way through the button 61. In this case theorientation of the transient assembly button on the end cap is notimportant.

[0041] For thicker transient assembly buttons, grooves and notches canbe utilized in various combinations to achieve an improved ease ofremoval.

[0042] While there has been shown and described what are at the presentconsidered the preferred embodiments of the invention, it will beobvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

We claim:
 1. A ceramic arc tube assembly comprising: a hollow bodyhaving at least one open end containing an end cap, the end cap having acapillary tube and a transient assembly button, the capillary tubeextending outwardly from the hollow body and having a length insertedinto the open end and forming a seal with the hollow body, the transientassembly button being fixed around the capillary tube and contacting anedge of the open end, the interaction between transient assembly buttonand the edge of the open end determining the length of the capillarytube inserted into the open end; and the transient assembly button beingcapable of removal from the capillary tube without causing damage to thearc tube assembly.
 2. The ceramic arc tube assembly of claim 1 whereinthe periphery of the transient assembly button extends beyond theperiphery of the open end.
 3. The ceramic arc tube assembly of claim 1wherein the transient assembly button is grooved to facilitate removal.4. The ceramic arc tube assembly of claim 1 wherein the transientassembly button is notched to facilitate removal.
 5. The ceramic arctube assembly of claim 3 wherein the groove faces the open end.
 6. Theceramic arc tube assembly of claim 1 wherein the transient assemblybutton is an annular disc.
 7. The ceramic arc tube assembly of claim 6wherein a surface of the transient assembly button is grooved along adiameter.
 8. The ceramic arc tube assembly of claim 7 wherein thegrooved surface of the transient assembly button faces the open end. 9.The ceramic arc tube assembly of claim 7 wherein the depth of the grooveis from about 50 percent to about 75 percent of the thickness of thetransient assembly button.
 10. The ceramic arc tube assembly of claim 6wherein the transient assembly button is notched.
 11. The ceramic arctube assembly of claim 6 wherein the diameter of the transient assemblybutton is greater than an outer diameter of the open end.
 12. Theceramic arc tube assembly of claim 1 wherein the length of the capillarytube inserted into the open end has a radially extending sealing memberwhich forms the seal with the hollow body.
 13. The ceramic arc tubeassembly of claim 1 wherein hollow body has two opposed open ends eachcontaining an end cap.
 14. A ceramic arc tube assembly comprising: ahollow cylindrical body having at least one open end containing an endcap, the end cap having a capillary tube and a transient assemblybutton, the capillary tube extending outwardly from the hollow body andhaving a length inserted into the open end, the length inserted into theopen end having a radially extending sealing member forming a seal withthe hollow body, a transient assembly button being fixed around thecapillary tube and contacting an edge of the open end, the interactionbetween transient assembly button and the edge of the open enddetermining the length of the capillary tube inserted into the open end;and the transient assembly button being capable of removal from thecapillary tube without causing damage to the arc tube assembly.
 15. Theceramic arc tube assembly of claim 14 wherein the transient assemblybutton is an annular disc.
 16. The ceramic arc tube assembly of claim 15wherein a surface of the transient assembly button is grooved along adiameter.
 17. The ceramic arc tube assembly of claim 16 wherein thegrooved surface of the transient assembly button faces the open end. 18.The ceramic arc tube assembly of claim 16 wherein the depth of thegroove is from about 50 percent to about 75 percent of the thickness ofthe transient assembly button.
 19. The ceramic arc tube assembly ofclaim 14 wherein the transient assembly button is notched.
 20. Theceramic arc tube assembly of claim 15 wherein the diameter of thetransient assembly button is greater than an outer diameter of the openend.
 21. The ceramic arc tube assembly of claim 14 wherein hollow bodyhas two opposed open ends each containing an end cap.
 22. The ceramicarc tube of claim 14 wherein the sealing member is formed as an integralpart of the capillary.
 23. A method of making a ceramic arc tubecomprising the steps of: (a) fixing a transient assembly button around acapillary tube to form an end cap; (b) inserting the end cap into anopen end of a hollow arc tube body until the transient assembly buttoncontacts an edge of the open end; (c) heating the assembly to form amechanical seal between the capillary tube and the open end of thehollow body; (d) removing the transient assembly button without damagingthe assembly; and (e) sintering the assembly to form the ceramic arctube.
 24. The method of claim 23 wherein the transient assembly buttonis fixed to the capillary tube by heating at or below about 1350° C. 25.The method of claim 23 wherein the assembly is heated at or below about1350° C. to form the mechanical seal.
 26. The method of claim 23 whereinthe hollow body in step (b) has two opposed open ends having end capsinserted therein and in step (c) the capillary tube of each end cap issealed simultaneously to the respective open end.
 27. The method ofclaim 25 wherein the assembly is sintered at a temperature above about1800° C. in a hydrogen-containing atmosphere.
 28. A method of making aceramic arc tube comprising the steps of: (a) fixing a transientassembly button and a sealing member around a capillary tube to form anend cap; (b) inserting the sealing member of end cap into an open end ofa hollow arc tube body until the transient assembly button contacts anedge of the open end; (c) heating the assembly to form a mechanical sealbetween the sealing member and the open end of the hollow body; (d)removing the transient assembly button without damaging the assembly;and (e) sintering the assembly to form the ceramic arc tube.
 29. Themethod of claim 28 wherein the transient assembly button is fixed to thecapillary tube by heating at or below about 1350° C.
 30. The method ofclaim 29 wherein the assembly is heated at or below about 1350° C. toform the mechanical seal.
 31. The method of claim 30 wherein theassembly is sintered at a temperature above about 1800° C. in ahydrogen-containing atmosphere.
 32. The method of claim 23 wherein thecapillary tube is subjected to a thermal pretreatment to densify thecapillary tube prior to step a).
 33. The method of claim 28 wherein thecapillary tube is subjected to a thermal pretreatment to densify thecapillary tube prior to step a).